The recipe

Trim 4 young leeks, discarding the toughest part of the green and slice them into 3cm rounds. Wash thoroughly under running water to remove grit. Put the sliced leeks in a large pot together with 30g of butter and 3 tbsp of chicken or vegetable stock. Place a piece of baking parchment or greaseproof paper over the top of the leeks, pushing it down into the pan so it touches them. Place a lid on top and cook over a moderate heat for 5 minutes till the leeks have softened.

Cut 1kg of floury potatoes, such as Maris Piper, into small, 2cm cubes, then add three-quarters of them to the softened leeks. Pour in 1 litre of vegetable or chicken stock, add 3 bay leaves and a little salt and bring to the boil. Lower the heat and simmer for about 20 minutes until the potatoes are soft.

In a shallow pan, warm 2 tbsp of oil, then fry the reserved potato cubes until tender and golden. Remove the potatoes from the pan and put them on kitchen paper. Place 4 small mackerel fillets in the pan, fry quickly, turning once, until they are golden and the skin side is lightly crisp.

Ladle half of the soup into a blender or food processor and process until smooth and velvety then stir back into the rest of the soup. Ladle the soup into warm bowls, divide the fried potatoes between them, then place a fillet of mackerel on each. Serves 4.

The trick

I find a stick blender useful for the soup, with less to wash up than a food processor or jug blender. You need not remove half the soup, as suggested above, but simply work the stick blender briefly through the soup, leaving the texture partially creamy.

The twist

I suggest mackerel here for its affinity with potatoes and leeks, but also for its economy. Most of the white fish will work, too, but pieces of smoked haddock are always welcome. You could simmer the fillets in a little milk and water to keep them moist, or lower them into the soup and let them cook for 4-5 minutes.